I would like to know how gamma rays from cosmic rays are blocked by the earth's atmosphere. I know that the sun's gamma radiation don't usually reach the earth so I wouldn't be worrying much about it. I have did some research and I only see that ozone will block uv radiation but not gamma radiation. So I am wondering how is gamma radiation being blocked by the atmosphere? What molecules in the atmosphere specifically absorb gamma radiation? Thank you!
[Physics] How is gamma ray blocked in the atmosphere in cosmic rays?
gamma-rays
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That's a good question and I think the answer may surprise you. It turns out that indeed, there's a lot of gamma ray radiation being produced in the sun's core from fusion reactions, so why are we not bombarded by gamma ray radiation? Those gamma ray photons need to escape from the sun's core, into the outer edge, and then finally from the surface. These photons are colliding with matter constantly, resulting in a photon diffusion process. It actually takes the average gamma ray photon about 170,000 years to diffuse out of the radiative zone. Once a gamma ray actually diffuses outward to the surface as a result of the constant collisions it has been converted into millions of visible wavelength photons.
more here: Solar Core Wiki
This is how the typical spectrum should look. 
However, your crystal can be quite bad and you see like one large bump. I remember old crystals having 700 keV resolution. Nothing to do.
Or
the tension is too low
the MCA is not properly set and maximum of the signal is not recorded (gate)
if you have a digital acquisition, you may need lot of work and understanding to tune correctly
Edit: 1. the picture is an illustrative one taken from i.stack.imgur.com/n0BzJ.jpg. Purpose is to give a hint how such a spectrum can look. The author of the question did not see anything reasonable up to now, nor he posted a picture.
The labeling is not mine, Compton edge label should be also at 1000 keV near the photopeak.
I modified labeling a picture a little bit.
Best Answer
Cosmic gamma rays are largely absorbed in the atmosphere by three ionizing interactions with atmospheric molecules: photoelectric effect, Compton scattering and pair production. Therefore, in order to observe cosmic gamma rays for astronomical purposes, gamma ray observatories (like the Compton Gamma Ray Observatory) have to be placed in space.